Inks and Washes vary, I've used some 'inks' that work like GWs new 'washes', Windsor and Newton's come to mind. Vice versa I have seen some 'washes' that looked a little like GWs 'inks'.
If we use the GW products as a basis then the difference is roughly:
> GW inks (discontinued): contain a lot of pigment (the colour) and have a very runny medium (the bulk) with a low surface tension. In practice this means when you apply it the ink collects in the deep parts of a model. Due to the low surface tension inks tend to leave a dirty look on flat surfaces (the ink dries unevenly) which often needs tidying up. Adding some matt varnish to an ink can go some way towards stopping this.
> GW Washes: These contain less pigment than the inks and have a very thick (resin like) medium. In practice this means the washes still collect in the deeper parts of the model but tend to still cover most of the rest of the model to some extent. As they dry they create a gradient between the parts with most Wash and the parts with the least. This gives a much tidier look that normally doesn't need tidying up. Washes do how ever tend to darken the whole model (unlike inks) meaning you have to paint the colour to be washed slightly brighter than you want it to end up.
